1. Field of the Invention
The present invention concerns a method of forming an image capable of providing a satisfactory gloss characteristic, image quality and curling characteristic, as well as a recording medium used for the method, for example, in a printer and a copying machine adopting a simultaneous transferring/fixing method of transferring a toner image formed by an indirect electrophotographic process on a recording medium and fixing the toner image upon transfer on the recording medium.
2. Statement of the Related Art
In recent years, preparation of color images has been conducted vigorously by using a digital indirect electrophotographic process. The digital indirect dry electrophotographic process forms an image, generally, by electrostatically developing toners of each of yellow, magenta, cyan and black colors prepared by mixing colorants such as pigment and dye in a thermoplastic resin, on a light sensitive material as an image support addressed digitally by exposure light converted from image information into optical information, and electrostatically transferring onto a recording medium the toner images thus formed and then melting and fixing the images by heating and pressing.
The toner used in the indirect dry electrophotographic process has a particle diameter from 5 .mu.m to 12 .mu.m and is generally transferred by a weight of 0.3 to 1.2 mg/cm.sup.2 per color on the recording medium. The thermoplastic toner is transferred by 1 to 4 layers on the recording medium depending on the desired hue. While these kinds of toner are softened and melted during heating, not all of them are penetrated into paper as a usual recording medium even in a heated and pressed state, for example, in the transferring/fixing step and are formed on the recording medium being raised 5 .mu.m to 20 .mu.m.
FIG. 1 shows the relation between an input image area ratio and an image gloss of the image thus formed on the recording medium. FIG. 1 is a graph illustrating a result of measuring 75.degree. mirror glossiness according to JIS P 8142 for a magenta image formed on cast coat paper as high gloss coat paper (Enamel coat/manufactured by Yonago Kako Seishi Co.), J coat paper as medium gloss coat paper (manufactured by Fuji Xerox Co.) and non-coated common J paper as low gloss paper (manufactured by Fuji Xerox) as an recording medium by using line screen while varying an input image area ratio.
FIG. 2 shows an image profile for a 40% image area ratio portion measured by a three-dimensional surface roughness meter for the thus obtained image on the cast coat paper. As can be seen from FIG. 1 and FIG. 2, while the image has a relatively high gloss in a solid image area, incident light scatters widely in a half tone region or a highlighted region since the toner fixed portion (image area) disposed in accordance with the shape of lines or dots is raised in a convex form from the recording medium, so that when an image of relatively large density gradation such as a portrait image, a high gloss region and a low gloss region are mixed together in the image to give a feeling of incongruity. Furthermore, it has been known that the color reproducibility is lowered in such an uneven image due to the effect of random reflection on the surface of the image to result in an image of low sharpness. Furthermore, it has known that the surface uneven image on a transparent recording medium lowers color formation due to scattering of transmission light in the case of OHP projection as well.
In order to improve the quality of such a color image, Japanese Published Unexamined Patent Application No. Sho 62-92965 proposes a method of providing a transparent resin layer on a recording medium, transferring toner on the recording medium and then burying it into the transparent resin layer by a roll heat-fixing machine. In this method, however, an oil membrane of a low surface tension is formed between the toner and the thermoplastic transparent resin under the effect of a silicone oil as a releasing agent coated on a fixing heat roll, so that the toner is not effectively buried in the thermoplastic transparent resin layer to leave unevenness as it is on the surface.
For example, Japanese Published Unexamined Patent Application No. Hei 5-216322 proposes, for a similar purpose, a method of electrostatically transferring toner on a recording medium having, on the surface, a transparent resin layer made of a thermoplastic resin with a thickness from 20 to 200 .mu.m, and then burying the toner into the transparent resin layer with a belt fixing machine. In this method, the toners are cooled sufficiently till they are transported to a peeling position by use of the belt-like fixing machine, use of the releasing oil is not necessary since the self coagulation force of the toner is utilized upon peeling and, further, a sufficient heating time can be obtained in a transferring/fixing step. However, even if the toner and the thermoplastic transparent resin layer can be melted sufficiently by belt heating, in the combination of a general toner resin and the thermoplastic surface layer resin as described in the prior art publication, compatibility between them during melting is not sufficient and difference in the refractive index is caused at the interface in the surface coating layer to result in lowering of color reproducibility or leave a little unevenness. In addition, transfer of the toner image to the recording medium is conducted electrostatically in both of the prior art publications, and the dielectric constant of the thermoplastic resin disposed on the surface of the recording medium is low, so that the transfer ratio is lowered upon multiple transfer of toner of different colors, particularly, for the final transfer color, to cause color shadow or lower the gamut.
Further, in the case of a recording medium having a thermoplastic resin layer disposed on the surface of the substrate, since the thermal property is different between the substrate and the resin layer, the resin heated and melted during fixing shrinks slightly upon cooling and solidification, to result in a large curl over the entire recording medium.
As a method of preventing the recording medium from curling, U.S. Pat. No. 5,087,536 proposes to provide, on the opposite side of the substrate, a resin having a melting point higher than that of a thermoplastic resin disposed on the recording surface of the recording medium. However, this method is not only fails to provide a sufficient anti-curling effect but also has a worry of causing running failure due to frictional force between resins during paper feeding.